1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an electrophotographic image forming apparatus for forming color images and monochromatic images, such as a copying machine and a printer, etc.
2. Description of Related Art
Electrophotographic full-color image forming apparatuses are generally of a tandem type wherein toner images of three primary colors and black are formed at respective image forming stations, each of which comprises a photosensitive drum. The toner images formed at the respective image forming stations are transferred onto an intermediate member (first transfer), and a composite image resulting from the first transfer is transferred onto a transfer member (second transfer).
In color image forming apparatuses of this type, generally, the print mode is switchable between a color mode and a monochromatic mode. A color image forming apparatus of this type comprises a scanning optical system for forming images on the photosensitive drums, and the scanning optical system scans four laser beams on the four photosensitive drums for formation of respective color images and for formation of a monochromatic image. In forming an image on each of the photosensitive drums, it is necessary to align the starting points of writing lines in a main scanning direction. For simplification, one of the laser beams is selected to be used to time the starts of writing lines with respect to image formation on all the photosensitive drums.
Japanese Patent Laid-Open Publication No. 2004-9349 (Prior Art 1) teaches that the laser beam used to form an image in a subtractive color mode (monochromatic mode) is also used for timing start of writing (for synchronization of writing) in a color mode. Also, Japanese Patent Laid-Open Publication Nos. 2001-324688 (Prior Art 2) and 11-287964 (Prior Art 3) relate to control for stabilizing a laser beam incident to a start timing sensor. These publications teach that all the laser beam emissions for synchronization of writing in the main scanning direction are controlled independently of each other.
There is a problem in a scanning optical system as disclosed by the Prior Art 1 that in accordance with the range of change in the quantity of light required for image formation, the beam emission for synchronization of writing in the main scanning direction is changeable. Also, in a scanning optical system as disclosed by the Prior Arts 2 and 3, there is a problem that beam emission for synchronization of writing and beam emission for image formation must be designed differently from each other.